Thiolytic reaction

The 3-ketothiolase has been purified and investigated from several poly(3HB)-synthesizing bacteria including Azotobacter beijerinckii [10], Ral-stonia eutropha [11], Zoogloea ramigera [12], Rhodococcus ruber [13], and Methylobacterium rhodesianum [14]. In R. eutropha the 3-ketothiolase occurs in two different forms, called A and B, which have different substrate specificities [11,15]. In the thiolytic reaction, enzyme A is only active with C4 and C5 3-ketoacyl-CoA whereas the substrate spectrum of enzyme B is much broader, since it is active with C4 to C10 substrates [11]. Enzyme A seems to be the main biosynthetic enzyme acting in the poly(3HB) synthesis pathway, while enzyme B should rather have a catabolic function in fatty-acid metabolism. However, in vitro studies with reconstituted purified enzyme systems have demonstrated that enzyme B can also contribute to poly(3HB) synthesis [15]. 

Methanotrophic and methylotrophic metabolism may be expanded to include a set of oxidative, hydrolytic or thiolytic reactions whereby simple orgatuc structures can be transformed to the methanotrophic intermediates methanol, formaldehyde. 

In peroxisomes, membrane-enclosed organelles of animal and plant cells, the intermediates for /3 oxidation of fatty acids are coenzyme A derivatives, and the process consists of four steps, as in mitochondrial /3 oxidation (Fig. 17-13) (1) dehydrogenation, (2) addition of water to the resulting double bond, (3) oxidation of the /3-hydroxyacyl-CoA to a ketone, and (4) thiolytic cleavage by coenzyme A (The identical reactions also occur in glyoxysomes, as discussed below.)... 

Another related reaction that goes through a ketimine is the conversion of the amino acid kynurenine to alanine and anthranilic add.225 It presumably depends upon hydration of the carbonyl group prior to P cleavage (Eq. 14-35). An analogous thiolytic cleavage utilizes CoA to convert 2-amino-4-ketopentanoate to acetyl-CoA and alanine.226... 

The preceding reactions have oxidized the methylene group at C-3 to a keto group. The final step is the cleavage of 3-ketoacyl CoA by the thiol group of a second molecule of CoA, which yields acetyl CoA and an acyl CoA shortened by two carbon atoms. This thiolytic cleavage is catalyzed by -ketothiolase. 

Ketoacyl-CoA thiolase (/6-ketothiolase) catalyzes a thiolytic cleavage, has broad specificity, and yields acetyl-CoA and acyl-CoA shortened by two carbon atoms. The reaction is highly exergonic... 

In this reaction, sometimes called a thiolytic cleavage, an acetyl-CoA molecule is released. The other product, an acyl-CoA, now contains two fewer C atoms. 

Zn(SH)(Tp Bu>Me)] does not react with esters, phosphates, or CO2, e.g., for thiolytic cleavage reactions, whereas acidic organic X-OH compounds (carboxylic acids, trinitrophenol, hexafluoroacetylace-tone) replace the SH groups with the formation of [Zn(OX)-(Tp Bu Me)].103 The reaction of [Zn(OH)(Tp,Bu Me)] with alcohols has been followed by combined experimental and computational investigations in order to determine the factors influencing the formation of tetrahedral alkoxide complexes. The [Zn(OR)(Tp Bu,Me)l derivatives have been found unstable toward hydrolysis.104... 

The products of the reaction are acetyl-CoA and an acyl-CoA shortened by two carbon atoms. The equilibrium of the reaction is more to the side of the thiolytic cleavage products thereby driving P-oxidation to completion. All thiolases that have been studied in detail contain an essential sulfhydryl group, which participates directly in the carbon-carbon bond cleavage as outlined in the following equations where E-SH represents thiolase [4]. 

Inside of the mitochondrion, fatty acyl-CoAs are oxidized in a series of steps that each release a two-carbon fragment, in the form of acetyl-CoA. Each step involves four reactions-dehydrogenation, hydration, dehydrogenation, and thiolytic cleavage (Figure 18.16). The individual reactions are summarized as follows ... 

Thiolytic cleavage of the 3-ketoacyl-CoA is catalysed by acetyl-CoA acyl transferase (EC 2.3.1.16) which liberates acetyl-CoA and an acyl-CoA two carbons shorter than the original substrate (Fig. 11.7). The above enzyme has a broad chain-length specificity but a second, acetyl-CoA acetyl-transferase (EC 2.3.1.9), catalyses the same reaction with acetoacetyl-CoA as substrate. Kinetic and exchange studies indicate that these thiolase reactions occur in two stages (cf. Greville and Tubbs, 1968). 

The final process for coenzyme A thioester synthesis is by the thiolytic cleavage of /8-keto acyl coen me A derivatives. The thiolase reaction is the principal metabolic process for degrading the hydrocarbon chain of fatty acids. 

This procedure can be regarded as conplementary to the thiolytic cleavage of the Dts imide protecting group. If, however, the reaction is carried out with triphenylphosphine at II0°C, under strictly anhydrous conditions, using toluene as solvent, the A-alkylated heterocycle is converted into the corresponding isocyanate 17 with concomitant formation of triphenylphosphine sulfide and carbonyl sulfide (eq 15) 4.. 13.14.24,27... 

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